1. Field of the Invention
This invention relates to photographic enlarging and printing and in particular to an integrator for spectral analysis of printing illumination from photographic film.
2. Description of the Prior Art
In the field of color photography, color correction of photographic film (including negatives and positive transparencies) is generally required to produce pleasing and realistic prints. Such correction is necessitated by such factors as imperfections in the photographic medium, variations in photographic processing, subject matter and the manner in which the photograph was taken. Corrections are achieved by varying the spectral content, (i.e. the amounts of blue, red, and green light) of the printing illumination. These variations cause corresponding changes in the cyan, magneta and yellow images respectively produced in the emulsion layers of the color printing paper.
Experience has shown that most color photographs have aggregate chromatic values which fall within certain "population centers" within a gray portion of a color spectrum. Occasionally photographs will appear having aggregate chromatic values outside these population centers, for example, a photograph primarily containing blue sky or water. Computer programs utilized in conjunction with color printing processes may be set to print most photographs within the population centers and yet accomodate those having aggregate primary colors outside the gray range of the spectrum. A variety of apparatus are well known for producing such variations in the spectral content of the printing illumination. For example, in a subtractive system, color filters are provided for passing desired amounts of light in selected color bands from a common strong white light source. A desired color composition is achieved by combining light from the respective filters.
Another type of light source, exemplified by my U.S. Pat. No. 4,124,292 is generally referred to as an additive lamp system. In an additive system, a plurality of lamps each producing one of the primary colors are employed. The output of each lamp is individually controlled to achieve light of proper composition which is mixed, for example in a mixing chamber, for application to the photographic film.
Another type of subtractive light source utilizes interference filters which are placed in the beam of a single light source, such as that from an enlarger lens, at successive predetermined times. The color dyes in the print paper emulsion thus receive proper amounts of exposure for realistic color images in the resulting print.
Regardless of the type of light source utilized with an enlarger or printer, means must be provided for analyzing the spectral content of the photographic film illumination to determine the proper color composition for printing. The photographic film illumination is generally defined by four parameters corresponding to the density (i.e., lightness or darkness) and the amounts of blue, red and green light in the printing illumination passing through the film. The parameters may be expressed in, for example, numeric values. The resulting numeric values may be either read by an operator who then manually sets the light source for a desired color composition or may be received by a computer coupled to the light source for automatically performing this function. With respect to the density parameter, subject classifiers are available for analyzing this value of the negative.
For analyzing the spectral content of the printing illumination, it is known to utilize a video analyzer which projects a color image on a screen for overall color evaluation whereby the inputs for the light source may be determined by an operator. For example, a cathode ray tube or a flying spot scanner may be utilized to illuminate a color negative and obtain a spectral analysis thereon. However, such video analyzers are relatively complex and expensive and require attendance by an operator.
Spot analyzers may also be employed which check the spectral content of a particular part of the film, for example a person's face. Such analyzers are most useful in conjunction with portraits and are not well adapted for use with other types of photographs.
Total analyzing is a method of integration wherein the entire color spectrum of a negative is considered. One type of apparatus employed therewith has spectrally distinct photocells which respond to discreet primary colors within the printing illumination from the film. Such systems are also referred to as "integration to gray" systems because the light source is set to produce a printing illumination falling within a predetermined "population center" on the color spectrum wherein the total color composition has a gray value. Such photocells may be mounted in photo multiplier tubes which are radially arranged around an enlarger lens to receive the printing light from illuminated film for analysis.
However, such photocell and tube systems have a significant drawback in that they require significant amounts of space. In particular, the space limitations present difficulties when relatively small negatives (e.g., eleven millimeter film) are to be printed at a relatively large size, such as eight inches by ten inches. In such applications, the printer lens is very close to the film, whereby photocell multiplier tube systems are generally precluded by the space limitations. Further, such systems comprise a relatively complex array of photocells and tubes, necessitating a relatively high cost.
Heretofore, there has not been available an integrator with the advantages of photocell tubes at substantially less cost and adaptable to both enlargers and printers and compatible with various sizes of films and prints.